The present invention relates to a reversible motor driving circuit capable of controlling a motor used for a power window etc. so as to rotate in forward and reverse directions.
FIG. 3 is a circuit diagram showing a related reversible motor driving circuit for rotating a motor in forward and reverse directions. The driving circuit shown in FIG. 3 uses two mechanical relays RLY1 and RLY2 for rotating a motor M in forward and reverse directions. The reversible motor driving circuit includes a CPU 10 which operates by being supplied with a logic power source VDD of 5 volt, for example, to control the motor M, an input I/F (interface) 20 for inputting an ON/OFF signal and a forward/reverse rotation signal etc. from a switch SW to the CPU 10, and a multiplex I/F 30 for inputting an ON/OFF signal and a forward/reverse rotation signal etc. transmitted from other electronic units through a LAN (local area network) etc. to the CPU 10.
The reversible motor driving circuit further includes a transistor Tr3, the mechanical relay RLY1 for the forward rotation, a transistor Tr4 and the mechanical relay RLY2 for the reverse rotation. Diodes D6 and D7 prevent reverse currents from flowing into the coils of the mechanical relays RLY1 and RLY2, respectively. Diodes D8 and D9 are free-wheel diodes for bypassing currents flowing into the collectors of the transistors Tr3 and Tr4 at the time of turning-off states of the transistors Tr3 and Tr4, respectively.
In such a circuit, in the case of rotating the motor M in the forward direction by the signal from the input I/F 20 or the multiplex I/F 30, the CPU 10 turns on the base of the transistor Tr3 and turns off the base of the transistor Tr4. As a result, the mechanical relay RLY1 is turned on and the mechanical relay RLY2 is turned off so that the forward current flows into the motor M. In contrast, in the case of rotating the motor M in the reverse direction, the CPU 10 turns off the base of the transistor Tr3 and turns on the base of the transistor Tr4. As a result, the mechanical relay RLY1 is turned off and the mechanical relay RLY2 is turned on so that the reverse current flows into the motor M. Further, in the case of stopping the motor M, the transistors Tr3 and Tr4 are turned off thereby to turn off the mechanical relays RLY1 and RLY2.
A shunt resistor R1 and a lock current detection circuit 50 are required for a load being locked such as a power window etc. of a vehicle. When a lock current representing a lock state of the motor M is detected, the lock current detection circuit 50 notifies the detection to the CPU 10, and the CPU 10 performs such a control of stopping the rotation of the motor M.
JP-A-7-251749 discloses a motor voltage detection circuit for detecting the ground fault of a motor or the abnormality of a motor driving device by using an H bridge circuit in a circuit for driving the motor.
The related reversible motor driving circuit using the mechanical relays shown in FIG. 3 has the following problems. (1) Since the mechanical relays are used, it is difficult to reduce the size and the weight of the reversible motor driving circuit. (2) The mechanical relays generate sound when their contact points close and open. (3) When the mechanical relay is turned off (when the contact point thereof opens), noise is generated due to an arc and so the CPU etc. may be badly influenced. (4) There is a limit in the durable number of times as to the contact points of the mechanical relays. (5) Since a large current continues to flow in the mechanical relay at the time of shortage of an electric wire or a load, the contact point may be fused.
These problems can be solved by using semiconductor elements as a switching driving device disclosed in JP-A-7-251749. However, in the semiconductor element such as an FET, the element is likely broken due to an excess current and hence it is required to provide a device for protecting the element from the excess current. In this respect, although JP-A-7-251749 provides a circuit for protecting the elements from the ground fault of the motor etc., there is no disclosure as to the protection of the semiconductor element etc. in the case of reversely connecting a DC power source such as a battery.